Electronic smoking device

ABSTRACT

An electronic smoking device comprises a housing accommodating a battery ( 10 ) as an electric power source powering an electrically heatable atomizer comprising an electric heater ( 22 ) and adapted to atomize a liquid supplied from a reservoir to provide an aerosol exiting from the atomizer. The heater ( 22 ) of the atomizer is controlled by means of control electronics ( 14 ). A puff detector ( 18 ) indicates an aerosol inhaling puff to the control electronics ( 14 ). The control electronics ( 14 ) operates, for a puff, the heater ( 22 ) of the atomizer with a pre-determined level of electric power. Preferably, the electronic smoking device comprises a voltage sensor ( 34 ) measuring the battery voltage, wherein the control electronics ( 14 ) distributes the pre-determined level of electric power, for a puff, by pulse width modulation, based on the battery voltage measured by the voltage sensor ( 34 ).

The invention relates to an electronic smoking device, in particular anelectronic cigarette.

An electronic smoking device, e.g. designed as an electronic cigarette,generally comprises a housing accommodating an electric power source(usually a battery or a rechargeable battery), an electrically heatableatomizer including an electric heater adapted to atomize a liquidsupplied from a reservoir (usually a capsule) in order to provide anaerosol exiting from the atomizer, and control electronics which controlthe heater of the atomizer. A puff detector indicates or signals anaerosol inhaling puff to the control electronics. Usually, the puffdetector is designed as an inhaling sensor detecting a user's puff, butit can also be accomplished, e.g., as a simple push button pressed bythe user while inhaling the aerosol. When a puff is indicated to thecontrol electronics, the heater in the atomizer is powered, which causesthe creation of aerosol. Here and in the following, the action of theatomizer is called “atomize” and the related product is called“aerosol”, irrespective of its composition, which might include gaseousand smoke constituents.

EP 2 443 946 A1 discloses an electronic cigarette and a capsulecontaining a liquid to be atomized (or evaporated) in an atomizer. Thecapsule comprises a shell which is sealed at one end side by apuncturable membrane. To mount the capsule at the mouth-ended side ofthe electronic cigarette, a soft sleeve surrounding the capsule isplaced on the end area of a tube accommodating the atomizer. During themounting step, a spike provided at the end of a kind of metal, wickpierces the membrane, and the liquid of the capsule is guided by thewick to the atomizer. The aerosol generated by the atomizer passes thearea of the capsule through some ducts provided at the exterior surfaceof the capsule to reach an end opening where it can be inhaled by theconsumer.

US 2011/0304282 A1 describes a power supply section for an electroniccigarette. This section comprises an elongate housing sleeve, whichaccommodates a rechargeable battery, an inhaling sensor for detecting anaerosol inhaling puff of a user, and control electronics connected tothe inhaling sensor and adapted to control the heater of an atomizer. Atan end side of the housing sleeve, a connector provides a mechanicalsupport to a mouth-sided section of the electronic cigarette, whichcomprises the atomizer and holds a capsule containing a liquid to beatomized. The connector includes electrical connections for theatomizer.

In the known electronic smoking devices, the heater of the atomizer isgenerally driven by the voltage provided by the battery and powered fora certain time period, usually the period during which the inhalingsensor signals the presence of a puff or for a pre-determined period.When the battery is discharged during use, its voltage drops so that thepower of the heater decreases, which results in less intensive puffs. Insome devices, the user can influence the heater by external settings,which might affect the aerosol emerging from the atomizer in anundesired manner, however. Generally, when the heater is not hot enough,aerosol creation is poor or stops completely. On the other hand, whenthe heater gets too hot, too much aerosol is created or, even worse,aerosol substances may burn or disintegrate, and the battery isdischarged too quickly. It would be desirable to measure the temperatureof the heater, but this would require a temperature sensor close to theheater, implying an appreciable technical effort.

U.S. Pat. No. 6,040,560 discloses a power controller for an electricalsmoking system which externally heats a tobacco-containing cigarette.This controller uses a power cycle including at least two phases, eachhaving a predetermined total energy input.

The object of the invention is to provide an electronic smoking device,in which the heater of the atomizer can be operated in a simple,reliable and reproduceable manner, largely independent of changes of thebattery voltage.

This object is achieved by an electronic smoking device having thefeatures of claim 1. Advantageous embodiments of the invention followfrom the dependant claims.

The electronic smoking device according to the invention comprises ahousing, which accommodates a battery as an electric power sourcepowering an electrically heatable atomizer. The atomizer comprises anelectric heater and is able to atomize a liquid supplied from areservoir to provide an aerosol exiting from the atomizer. Moreover, theelectronic smoking device comprises control electronics and a puffdetector. The puff detector, for example an inhaling sensor or amanually actuatable switch (see below), indicates an aerosol inhalingpuff to the control electronics. The control electronics are adapted tocontrol the heater of the atomizer. For example, when the puff detectordetects a puff and indicates or signals that to the control electronics,the heater is powered by means of the control electronics in order tocreate aerosol.

According to the invention, the control electronics operates, for apuff, the heater of the atomizer with a pre-determined level of electricpower. That means, the electric power (i.e. electric energy per timeunit) supplied to the heater is controlled in a way so that it ismaintained at the pre-determined level for the actual puff. This doesnot exclude fluctuations of the power during the puff, provided thefluctuations are on a time scale which is short compared to the durationof the puff. In other words, the electric power by which the heater isoperated is kept constant at the pre-determined level, during the puff,to a rather high degree of accuracy. Thus, the conditions during aninhaling puff are largely reproducable, even when the battery voltagedrops due to consumption and discharging. Given the latent heat foraerosol production in the atomizer and the thermal losses due toradiation and convective heat transport by means of the aerosol, thepre-determined power level may correspond to a more or less well-definedtemperature of the heater and the atomizer. Such a general relationbetween power and temperature permits a pre-selection of a suitablepower level, which in practice is preferably performed experimentally.

A complete electronic smoking device according to the invention, forexample an electronic cigarette, may include components in addition tothe housing, the puff detector and the control electronics mentionedabove. For example, the atomizer and the reservoir may be accommodatedin a separate section connected to the above housing. However, perdefinition the term “electronic smoking device” is also used for adevice just including the housing, the puff detector and the controlelectronics because that device is related to electronic smoking andmight be marketed separately. In this sense, the battery (which ismounted in the housing, but which usually is replaceable), the atomizerand/or the reservoir may be or may be not components of the electronicsmoking device.

In advantageous embodiments, the electronic smoking device according tothe invention comprises a voltage sensor. The voltage sensor measuresthe battery voltage and is connected to the control electronics.Moreover, the control electronics is adapted to distribute thepre-determined level of electric power, for a puff, by pulse widthmodulation, based on the battery voltage measured by the voltage sensor.

For example, the voltage sensor detects when the battery voltage dropsin the course of time. Based on that information, the controlelectronics is able to adjust the duty cycle of thepulse-width-modulated voltage provided to the heater terminals. Assumingthat the electrical resistance of the heater is largely constant, i.e.independent of temperature (see below) and has a pre-determined value,an instantaneous voltage level at the heater terminals corresponds to acertain instantaneous current flowing through the heater(current=voltage/resistance) and thus to a certain instantaneouselectrical power dissipated in the heater resistance(power=voltage×current=voltage²/resistance). The voltage supplied to theheater terminals can be the battery voltage. Thus, the pre-determinedlevel of electric power can be maintained as long as it is smaller than(battery voltage)²/resistance, and the control is performed via the dutycycle of the pulse width modulation. This will be explained in moredetail by means of an embodiment, see below.

If the electrical resistance of the heater is not constant, pulse widthmodulation may be used as well. In any case, a look-up table determiningthe relation between measured battery voltage and duty cycle can beestablished experimentally, for a pre-determined electrical power.

Due to the internal resistance of the battery, the battery voltageavailable at the terminals of the battery depends on the current flowingthrough the heater. Since the current is switched on and off in pulsewidth modulation, the battery voltage will change during a puff.Additionally, when no current is flowing, the battery voltage recoversas a function of time, which also depends on the charge state of thebattery.

In practice, the battery voltage fluctuates during a puff, which createsthe problem of selecting appropriate values for the battery voltage inorder to run the pulse width modulation in a reliable manner.

In advantageous embodiments of the invention, the control electronics isadapted to receive multiple measured values of the battery voltageduring a puff and to derive a representative value of the batteryvoltage therefrom. This representative value is taken as an appropriatevalue for the battery voltage, which can be used for determining theduty cycle of the pulse width modulation. In principle, therepresentative value of the battery voltage might be selected by ananalogue technique, e.g. by filtering, which takes some average over thefluctuations.

A digital technique may be more advantageous, however. For example, thecontrol electronics can be adapted to derive the representative value ofthe battery voltage as the smallest individually measured value of thebattery voltage plus a constant. In other words, the battery voltage isfrequently measured during a certain puff (puff n), and the results arestored. Lower measured values tend to be associated with the heaterbeing switched on and are representative for the battery under load. Onthe other hand, the lowest value of the battery voltage will not berealistic because of fluctuations and short-time recovery of thebattery. Therefore, a constant is added, for example a constant of 200mV.

The representative value of the battery voltage derived in this wayduring puff n can be applied in the next puff (i.e. in puff n+1) as thevalue for the battery voltage. That means, this value is taken as aconstant value for the battery voltage during puff n+1 for the purposesof determining the pulse widths to observe the pre-determined level ofelectric power during puff n+1. During puff n+1, the battery voltage canbe frequently measured as well, as in puff n, to derive a newrepresentative value of the battery voltage, which is to be used in puffn+2, and so on. In this way, during each puff, the battery voltage istaken as a constant, but it is considered that the battery voltage willchange with increasing puff number. For the first puff (puff 1), e.g.the first puff after the battery has been freshly charged, an empiricalvalue can be taken as the representative value of the battery voltage,e.g. the nominal voltage (highest voltage) of a lithium ion batteryminus an empirical correction constant.

In other embodiments, the control electronics receives multiple measuredvalues of the battery voltage during a puff and re-adjusts the pulsewidths (duty cycle) during a puff to observe the pre-determined level ofelectric power. That means that the control electronics is able to reacton changes of the battery voltage during a puff and to adjust the dutycycle accordingly still during this puff.

The voltage sensor can be integrated in the control electronics. It mayalso be provided as an independent voltage meter. A person skilled inthe art knows many possibilities for accomplishing a voltage sensor formeasuring the battery voltage and transmitting the result to the controlelectronics. For example, the battery voltage can be measured by usingan ADC (analog-to-digital converter) either integrated in amicrocontroller (system on chip) or provided as an independent IC(integrated circuit) that is connected through an I2C (interintegratedcircuit bus) or SPI (serial peripheral interface bus) interface to themicrocontroller or system on chip.

There are several options for the kind of response initiated when a puffis indicated to the control electronics. In an advantageous embodiment,the control electronics is adapted to operate, upon indication of apuff, the heater of the atomizer with the pre-determined level ofelectric power as long as the puff is indicated. That means, the amountof energy delivered to the heater (and thus the amount of aerosolcreated) is variable and depends on the length of the puff. In anotherembodiment, the control electronics is adapted to operate, uponindication of a puff, the heater of the atomizer with the pre-determinedlevel of electric power for a pre-determined period. In that case,indication of a puff just triggers a pre-determined action, i.e. thedelivery of a pre-selected amount of electrical energy to the atomizer.

In advantageous embodiments of the invention, the control electronicsoperates the heater of the atomizer with a reduced level of electricpower, when it is not operated with the pre-determined level of electricpower. That means, in the intervals between the puffs, the heater is notswitched off, but run at a lower temperature, which avoids that theheater cools down too much so that it can be quickly re-activated upondemand. On the other hand, the overall energy consumption is higher inthis operating mode.

As already explained further above, the puff detector transmits(indicates) to the control electronics the message that the user takesan inhaling puff. To this end, a manually actuatable switch (e.g. a pushbutton) is sufficient, which is pressed by the user during inhaling. Ina more elaborate design, the puff detector comprises an inhaling sensor,e.g. a sensor detecting a vacuum or an air flow in the housing of theelectronic smoking device, which is created when the user is inhaling ata mouthpiece.

In advantageous embodiments of the invention, the heater of the atomizercomprises a heating resistor, wherein the material of the heatingresistor comprises a nickel-chromium alloy, a nickel-chromium alloycomprising 80% per weight nickel and 20% per weight chromium (NiCr alloy80/20), a film polyimide, and/or an iron-chromium-aluminium alloy(FeCrAl alloy, Kanthal alloys). The temperature dependence of theelectrical resistivity is relatively low for these alloys, whichfacilitates the control of the atomizer.

The battery can be a component of the electronic smoking device.Preferably, the battery is designed as a re-chargeable lithium ionbattery protected by safety circuitry. In this case, the controlelectronics described so far does not monitor the charge-state of thebattery in order to prevent the battery from completely discharging orfrom over-charging. Such safety-relevant tasks are performed by thesafety circuitry, which may be designed as a part of the battery.

In the following, the invention is explained in more detail by means ofexamples. The drawings show in

FIG. 1 a schematic longitudinal section of an embodiment of theelectronic smoking device according to the invention,

FIG. 2 a block diagram of the electronics of the embodiment according toFIG. 1, and

FIG. 3 a graphical representation of the voltage applied to an electricheater of an atomizer of the embodiment according to FIG. 1 as afunction of time (pulse width modulation).

FIG. 1 illustrates an embodiment of an electronic smoking device 1 in aschematic longitudinal section.

The electronic smoking device 1 comprises a cylinder-like housing 2 anda mouthpiece 4, which is designed as a detachable cap. Taking off themouthpiece 4 provides access to a replaceable capsule 6, which serves asa reservoir for a liquid.

The housing 2 accommodates a battery 10. In the embodiment, the battery10 is designed as a re-chargeable lithium ion battery and includes itsown safety circuitry. The battery 10 is connected, via leads 12 and 13,to control electronics 14, which includes integrated circuits mounted ona printed circuit board 15. The printed circuit board 15 also supports aplurality of light-emitting diodes (LEDs) 16, which are assembled behindrespective windows provided in the housing 2 and indicate the currentstatus of the electronic smoking device 1.

A puff detector 18 is connected to the control electronics 14. In theembodiment, the puff detector 18 is designed as an inhaling sensor,which detects the vacuum generated inside the housing 2 when a userinhales at the mouthpiece 4.

An atomizer 20 comprises a heater 22 connected via leads 23 to thecontrol electronics 14. The heater 22 includes a heating wire mounted ata ceramics shell (not shown in the Figures), which also supports a wickdevice 24 made of braided metal or sponge-like metal material. Apiercing tip 25 at the distant end of the wick device 24 is able topenetrate a membrane 26 used for sealing the capsule 6 so that liquid 28contained in the capsule 6 can be guided out of the capsule 6 andthrough the wick device 24 to the area of the heater 22.

At its free end, the mouthpiece 4 comprises an inhaling aperture 30. Atthe opposite end of the electronic smoking device 1, a charging port 32is provided which permits re-charging of the battery 10, e.g. via a USBport.

To use the electronic smoking device 1, a consumer inserts a freshcapsule 6 so that its membrane 26 is pierced and liquid is supplied fromthe capsule 6 via the wick device 24 to the area of the heater 22. Whenthe consumer inhales at the inhaling aperture 30, the puff detector 18senses the resulting vacuum inside the housing 2 and indicates that tothe control electronics 14. In response thereto, the heater 22 ispowered so that its heating wire is able to atomize the liquid in itsproximity in order to create an aerosol, which is inhaled by theconsumer. In the embodiment, the heater remains switched on as long asthe puff detector 18 senses a vacuum.

FIG. 1 is a schematic illustration of the general set-up of theelectronic smoking device 1. The functional set-up of the electronicsmoking device 1 is shown in FIG. 2 in a more appropriate way, by meansof a block diagram of the electronics.

According to FIG. 2, the battery 10 is connected to the controlelectronics 14 via the leads 12 and 13. The puff detector 18 isdisplayed as a block connected to the control electronics 14, becauseits output signal is fed to the control electronics 14. The heater 22 isalso connected to the control electronics 14, by means of the leads 23.In the embodiment, the heater 22 comprises a heating resistor made ofwire of a nickel-chromium alloy (80% per weight nickel and 20% by weightchromium). Other materials for the heating resistor, which arewell-known in the art, are conceivable as well. The above nickelchromium alloy has the advantage of a low dependency of the electricalresistivity on temperature. So far, the components shown in FIG. 2 arepresent in FIG. 1 as well.

Moreover, FIG. 2 displays a voltage sensor 34, which is connectedbetween the leads 12 and 13 of the battery 10 and detects the actualbattery voltage. The output of the voltage sensor 34 is supplied to thecontrol electronics 14. In the embodiment, the voltage sensor 34 isdesigned as a component of the control electronics 14. The voltagesensor 34 comprises an analogue-to-digital converter (ADC) which outputsa numerical value representative for the actual battery voltage, whichcan be processed by a micro-processor which is part of the controlelectronics 14. Other embodiments of the voltage sensor 34 areconceivable as well.

During use of the electronic smoking device 1, the voltage of thebattery 10 drops due to discharging. This process is monitored by thevoltage sensor 34 so that the control electronics 14 is quasicontinuously informed on the charge state of the battery 10. In order tooperate the heater 22 of the atomizer 20, for each puff, with apre-determined level of electric power, the control electronics 14distributes the electric power supplied to the heater 22 by pulse widthmodulation, based on the battery voltage measured by the voltage sensor34. When the battery voltage changes, the duty cycle of the pulse widthmodulation is adjusted so that the resulting power is maintained at thepredetermined value.

In the embodiment, an appropriate value for the battery voltage in eachpuff is determined in the following way.

The control electronics 14 receives, via the voltage sensor 34, multiplemeasured values of the battery voltage during a puff, and these valuesare stored. A puff is typically considerably longer than a typicalmeasurement time for the battery voltage, which in turn is considerablylonger than the period of pulse width modulation. In the embodiment, themeasurements of battery voltage and the period of pulse width modulationare not synchronised to each other.

At the end of the puff (called “puff n”), a representative value of thebattery voltage is derived from the stored measured values. To this end,the control electronics 14 selects the smallest battery voltage valuemeasured during this puff and adds a constant thereto. Appropriatevalues for the constant can be determined empirically. In theembodiment, the constant is 200 mV.

The representative value of the battery voltage derived in this wayduring puff n is applied in the next puff (i.e. in puff n+1) as thevalue for the battery voltage. That means, this value is taken as thebattery voltage during puff n+1 for the purposes of determining thepulse widths (duty cycle) to keep the electric power delivered to theheater 22 during puff n+1 at the pre-determined constant level.

During puff n+1, however, the battery voltage is also frequentlymeasured, as in puff n, to derive a new representative value of thebattery voltage, which is to be used in puff n+2, and so on. In thisway, during each puff, the battery voltage is assumed to be a constant,but it is taken into consideration that the battery voltage will changewith increasing puff number, i.e. that it usually will decrease due todischarging of the battery.

For the first puff (puff 1), e.g. the first puff after the battery 10has been freshly charged, an empirical value can be taken as therepresentative value of the battery voltage, e.g. the nominal voltage(highest voltage) of a lithium ion battery minus an empirical correctionconstant.

When the puff detector 18 does not sense a vacuum inside the housing 2,the heater 22 is not powered. Alternatively, during these intervals theheater 22 may be operated at a reduced or even much reduced power levelin order to keep some elevated temperature at the atomizer 20, whichpermits a faster restart of the atomizer 20 when the next puff isdetected. Such a reduced power level can also be controlled by means ofpulse width modulation, by applying a different duty cycle.

In the following, a numerical example is given which explains theoperation of the heater 22 by means of pulse width modulation of thevoltage applied to the heater leads 23. In this example, the heater 22is considered as an Ohmian resistor having a constant resistance ofR=2.25 Ohms, neglecting any inductive or capacitive effects. Therequired duty cycle, as a function of the actual battery voltage (i.e.the representative value of the battery voltage described above), forachieving a constant heater power of P₀=3 W is determined by a simplecalculation. The results can be stored as a look-up table in a memory sothat a microprocessor of the control electronics 14 can select the dutycycle in response to a representative value of the battery voltage.Inductive or capacitive effects may be taking into consideration, forexample, by empirical corrections of the duty cycle values in thelook-up table.

FIG. 3 schematically illustrates the voltage pulses supplied to theheater 22 as a function of time. The time unit is relative, i.e. relatedto the period of one complete pulse. In the embodiment, this period isabout 4 ms, which is very short compared to the typical duration of onepuff (several seconds). During each pulse, either the actual batteryvoltage or zero voltage is supplied, and the respective duty cycle istaken from the look-up table. FIG. 3 shows the pulses for two differentbattery voltages, i.e. 3 V and 4 V.

Using Ohm's law (U=R·I, with battery voltage U, heater resistance R andheater current I), the power P dissipated in the heater 22 during theswitched-on state of one pulse is P=U·I=U²/R. This state lasts for afraction c of one pulse period. During the fraction (1−c), the voltageis switched off. Thus, c can be calculated by c·P=P₀ or c=P₀·R/U². Theduty cycle of the pulse is determined by c.

The following table provides c as a function of the battery voltage U.It is assumed that U drops from an initial value of 4.2 V to 2.9 V. Whenthe battery voltage gets too low, the battery 10 is disabled by its ownsafety circuitry.

Battery voltage U [V] c 2.9 0.803 3.0 0.750 3.1 0.702 3.2 0.659 3.30.620 3.4 0.584 3.5 0.551 3.6 0.521 3.7 0.493 3.8 0.467 3.9 0.444 4.00.422 4.1 0.402 4.2 0.383

In practice, the look-up table can be calculated and stored in finersteps.

1. An electronic smoking device, comprising: a housing (2) adapted toaccommodate a battery (10) powering an electric heater (22) of anatomizer (20) adapted to atomize a liquid (28) supplied from a reservoir(6) to provide an aerosol exiting from the atomizer (20); controlelectronics (14) adapted to control the heater (22) of the atomizer(20); a puff detector (18) adapted to indicate an aerosol inhaling puffto the control electronics (14); and the control electronics (14) isadapted to operate, for a puff, the heater (22) of the atomizer (20)with a pre-determined level of electric power.
 2. The electronic smokingdevice of claim 1 further comprising a voltage sensor (34) adapted tomeasure battery voltage and connected to the control electronics (14),and the control electronics (14) is adapted to distribute thepre-determined level of electric power, for a puff, by pulse widthmodulation, based on a battery voltage measured by the voltage sensor(34).
 3. The electronic smoking device of claim 2 with the controlelectronics (14) adapted to receive multiple measured values of thebattery voltage during a puff and to derive a representative value ofthe battery voltage from the multiple measured values of the batteryvoltage.
 4. The electronic smoking device of claim 3 with the controlelectronics (14) adapted to derive the representative value of thebattery voltage as the smallest individually measured value of thebattery voltage plus a constant.
 5. The electronic smoking device ofclaim 3 with the control electronics (14) adapted to apply therepresentative value of the battery voltage derived during the puff as aconstant value for the battery voltage when determining the pulse widthsduring a subsequent puff to observe the pre-determined level of electricpower.
 6. The electronic smoking device of claim 2 with the controlelectronics (14) adapted to receive multiple measured values of thebattery voltage during a puff and to re-adjust the pulse widths duringthe puff to observe the pre-determined level of electric power.
 7. Theelectronic smoking device of claim 2 wherein the control electronics(14) is adapted to determine a required pulse width modulation by usinga pre-determined, constant value of the electrical resistance of theheater (22).
 8. The electronic smoking device of claim 2 wherein thevoltage sensor (34) is integrated in the control electronics (14). 9.The electronic smoking device of claim 1 wherein the control electronics(14) is adapted to operate, upon indication of a puff, the heater (22)of the atomizer (20) with the pre-determined level of electric power fora pre-determined period.
 10. The electronic smoking device of claim 1wherein the control electronics (14) is adapted to operate, uponindication of a puff, the heater (22) of the atomizer (20) with thepre-determined level of electric power as long as the puff is indicated.11. The electronic smoking device of claim 1 wherein the controlelectronics (14) is adapted to operate the heater (22) of the atomizer(20) with a reduced level of electric power, when it is not operatedwith the pre-determined level of electric power.
 12. The electronicsmoking device of claim 1 wherein the puff detector comprises a manuallyactuatable switch.
 13. The electronic smoking device of claim 1 whereinthe puff detector (18) comprises an inhaling sensor.
 14. The electronicsmoking device of claim 1 wherein the atomizer (20) is a component ofthe electronic smoking device (1) and the heater (22) comprises aheating resistor, wherein the material of the heating resistor comprisesat least one of the following materials: a nickel-chromium alloy, anickel-chromium alloy comprising 80% per weight nickel and 20% perweight chromium, a film polyim-ide, an iron-chromium-aluminum alloy. 15.The electronic smoking device of claim 1 wherein the battery (10) is acomponent of the electronic smoking device and the battery (10) is are-chargeable lithium ion battery protected by safety circuitry.
 16. Theelectronic smoking device of claim 4 wherein the constant is 200 mV.